Book in two parts by Fyodor Dostoevsky. Written in 1868, it is considered a masterpiece of modern literature because of Fyodor Dostoeyevsky's psychological realism (keep in mind that Freud had yet to publish anything at this time).
The Idiot can sometimes be found mentioned alongside some of his better known works as Crime and Punishment, and the Brothers Karamazov.
The book deals with the life of Leo Mysjkin, the last member of a noble family. He is considered an idiot or imbecile due to his naive personality: He always thinks the best of people, and gives love indiscriminantly. While people generally like him for these reasons, they still think of him as an idiot at worst, or too good for this world at best.
Fyodor Dostoevsky attempted through The Idiot to convey his conception of Christ as a childlike figure, but it's also a story about the modern times, where naive people can't exist.
It's a good book, but if you're unfamiliar with Fyodor Dostoevsky then I'd recommend starting with something a bit lighter. Perhaps Notes from the Underground.